1. Field
This invention relates to ceramic coating methods, namely, methods for coating of ceramic on metal, of metal on ceramic, or of ceramic on ceramic; and more particularly relates to liquid-formed, ceramic coating methods with uniform ceramic metallizing compositions and specially graded, substantially defect-free bonded regions to produce reproducibly strong and thermomechanically shock-resistant coatings.
By ceramic I mean not only the usual ceramics such as alumina, zirconia, beryllia, mullite, cordierite, silicon carbide; but also quartz, intermetallics, diamond, boron, graphite, carbon, silicon, and various other carbides, nitrides, aluminides, or borides; glasses, machinable glasses, Corning's Vision glass; and the surface of many metals, particularly reactive metals such as aluminum, magnesium, chromium, silicon, titanium, or zirconium which always have oxides or other compounds of reactions of the metal with the environment.
2. Prior Art
Various methods have been developed to coat metal with, or to join metal to, ceramics. But none gives inexpensive, stable, strong, and temperature resistant products. Reliable ceramic coatings or joints are not commercially available worldwide at any cost.
Under a well-coordinated intensive effort on ceramic-metal bonding, Japan is the most successful country in the development and commercialization of products involving metal-ceramic bonds. They already have successfully: 1) used a ceramic turbocharger (NGK, Nissan), 2) produced an all ceramic swirl chamber for diesel engines (Mazda, NGK), and 3) prototyped a ceramic turbomolecular pump (Mitsubuishi and Japan Atomic Energy Research Institute), according to Prof. T. Suga of the University of Tokyo in his 1989 review paper on the "Future Outlook in Japan" (Copy enclosed). But the practical useful temperature of the best Japanese ceramic joints to special "matching" metal alloys is only 600.degree. C. Further, the bond strength decreases rapidly with temperature, because the reaction products in their bonded regions become weak and brittle under thermal stresses. They consider the improvement of the thermomechanical shock resistance of the joints to be an urgent task. The European effort, mainly in Germany and France, has been even less successful. Germany failed to reach their goal after the first ten-year (1974-1983) program and its follow-up in 1983-1986. Their present program ( 1985-1994) merely emphasizes on achieving reproducible mechanical properties and component reliability. The US Department of Energy supports much of US ceramic joining R&D. It also had to renew annually the ceramic automotive program after 10-year, 50-million intensive work.
Many problems still exist with present ceramic metallizing and bonding methods. A serious problem is the instability and unreliability of even the best ceramic-metal bonds, as mentioned above. Another problem is the difficulty of achieving uniform metallized layers formed on the ceramic. Take, for example, the commonly used heavy metal processes, such as W-Yttria (W-Y.sub.2 O.sub.3), W-Fe, or Mo-Mn. In these and many similar bonding methods, segregation of the mixed metal or other powders takes place due to their differing specific gravities, shapes, sizes, porosities, and surface smoothness. These segregation occur at all times: during the mixing of the powders, storing of the powder suspensions, application of the suspensions, settling of ths suspended powders in the applied coatings of the suspensions, and drying of the applied coatings. Further, these segregations occur so fast as to be practically uncontrollable, as will be shown shortly.
In general, spherical, heavy, large, smooth, and dense particles settle first and early in the binder or suspension medium. Upon settling, these particles tend to roll or move sidewise or downward toward the corners or boundaries faster and further than odd-shaped, light, small, rough, and porous particles of otherwise identical characteristics.
Take the W-Y.sub.2 O.sub.3 mixed powders in an organic binder of nitrocellulose in butyl carbitol acetate with specific gravities of 19.3, 4.5, and 0.98, respectively. Such a suspension, even if perfectly mixed up by shaking, stirring, roller-milling, or otherwise, will immediately tend to segregate. More specifically, the initial settling acceleration due to gravitational minus buoyancy forces on W powders is 980.6.times.(19.3-0.98)/19.3=930.8 cm.times.cm/sec, while that of Y.sub.2 O.sub.3 powders is only 767.0 cm.times.cm/sec.
In a mixing, storing, or carrying bottle 10 cm high and containing a perfectly mixed suspension of these metallizing powders, the time to completely settle out is only 147 ms (milliseconds) for W powders, if uniform acceleration is assumed. At the tip of a paint brush having a suspension drop 0.3 cm in diameter, the complete settling time of these same W powders is merely 25.4 ms, while on a horizontally painted or sprayed layer 0.1 cm thick, the same settling time is only 14.7 ms. In all these cases, the complete settling time for the Y.sub.2 O.sub.3 powders is always the square root of 930.8/767.0=1.21, or 21% longer.
Note in particular that the powder segregations with uniform accelerations may be completed within 147 to 14.7 ms. Such short times indicate that the W-Y.sub.2 O.sub.3 powder segregations are beyond human controls. Painted or sprayed mixed powder layers are thus always not uniform.
In metallizing onto a horizontal ceramic surface to be metallized, most of the W powders immediately settles out. The first layers are therefore always very rich in W (melting point 3,410.degree. C.), and correspondingly very poor in Y.sub.2 O.sub.3. These first layers are too refractory for the preset metallizing temperature (up to about 1550.degree. C.) so that the ceramic surfaces are not sufficiently metallized, or not at all. The last settling layers, on the other hand, are too rich in the fluxing Y.sub.2 O.sub.3. Again, the ceramic surfaces are improperly metallized, with only a glassy layer being formed which is very weak in strength and thermal or thermal shock resistance.
Thus, common metallizing results on ceramics are often erratic and uncontrollable. The metallized surface may contain loose and unmetallized spots with high heavy refractory metal content, as well as non-wettable spots due to the high flux content. Additional brushings and nickel or copper platings do not solve the basic problem. The entire process is critical and involved, and yet nonuniform. The resultant ceramic-metal joints or ceramic coatings on metals are weak, costly, nonreproducible, and usually not vacuum-tight, or temperature-resistant.
Painting or spraying onto vertical or inclined surfaces results in vertical and additional lateral segregations and gradations, and gives added poor uniformity, reproducibility, and bonding strength.
While only the effect of gravitational density segregation has been considered in some detail, the other segregation variables such as powder shape, size, porosity, and surface roughness are also important.
A second important problem with common joining processes is the lack of control, or even understanding, of dynamic mismatches of temperatures, stresses, and strain profiles in the joint region, and their variations with time. Another aspect of this invention is therefore to describe such dynamic mismatch phenomena, and to specially tailor-grade the composition and/or physical property profiles of the joint region so that the maximum or critical transient mismatch stresses never exceed the local material strength at any point inside the joint region, at any time during the heating or cooling of such joints in processing or service.
A third problem results from our incomplete understanding of the required microstructural, chemical, and physical properties of the interfacial regions in the ceramic-metal joints.
Accordingly, an object of this invention is to provide improved ceramic-metal joints and joining methods;
A further object of this invention is to provide improved ceramic metallizing methods for these joints;
A broad object of this invention is to minimize gravitational segregations of the components in the metallizing methods during or prior to the joining;
Another broad object of the invention is to specially tailor-grade, both in and normal to the joining plane, the composition and/or property profiles in the joint regions to ensure that the maximum dynamic or transient stresses do not exceed the local material strengths at any point and time;
A further object of the invention is to provide a specially microengineered interfacial region of the optimum characteristics to achieve defect-free, tough, and very strong joints;
Another object of the invention is to flawlessly coated metals or ceramics with protective materials;
A yet another object of the invention is to provide substantially flawlessly coated reinforcements for the manufacture of tough, strong, thermochemically stable, and thermomechanically shock-resistant composites;
Further objects and advantages of my invention will appears as the specification proceeds.